The present invention relates to apparatus for producing a continuous visual image of the physical conditions in the interior of a hot vessel. The technology of interest images hot interior surfaces where the surfaces are obscured by fume and radiation emitting/absorbing gases associated with a process taking place in the vessel. More particularly, the invention relates to imaging the interior of a wood pulp chemical recovery boiler.
In a manufacture of wood pulp, raw wood is digested in the presence of inorganic chemicals. In the sulfate or kraft pulping processes the active pulping chemicals are sodium hydroxide and sodium sulfide. An important aspect of kraft pulping is recovery of these inorganic pulping chemicals from the liquid pulping waste stream. This waste liquid, commonly called black liquor, is an aqueous mixture of lignin extracted from the wood and reacted inorganic pulping chemicals, principally sodium carbonate and sulfate. Economics of the overall pulping process require recovery of the relatively expensive pulping chemicals from the waste black liquor.
Conventionally, black liquor is combusted in a boiler designed to recover the pulping chemicals and extract thermal energy from the organic matter in the black liquor for steam production. The chemical recovery boiler receives black liquor after it has been concentrated in evaporators. The liquor is burned in the furnace and the chemicals are recovered as a molten smelt in the bottom of the boiler from which it discharges for recycling back into a fresh pulping liquor makeup system.
A principal function of the recovery process is the reconversion of sodium sulfate back to active sodium sulfide by the carbon residues in the boiler. Temperature and air flows are controlled to maintain a reducing zone in the boiler to change as much of the sodium sulfate as possible into sodium sulfide. The degree of reduction of sulfate to sulfide in the melt reflects the efficiency of the chemical recovery function and determines the quality of the chemical product from the recovery boiler.
The characteristics of the bed of molten solids and how they affect heat and chemical recovery are not known with certainty. This is largely due to the lack up until now of a means for accurately observing or measuring bed configuration. Fume and gaseous emissions from the smelt bed typically prevent significant continuous visual observation.
The bed itself is generally a very porous lattice containing approximately 5% carbon by weight. The typical smelt bed temperature is about 1000.degree. C. with overlying combustion gases at temperatures of 1100.degree.-1300.degree. C.
What is known suggests that smelt bed height on the floor of the furnace affects the efficiency of achieving the primary goal of reducing sodium sulfate to sulfide. In general, a medium bed height suggests good chemical reducing environment. On the other hand, a low bed height is thought to be safest, depending upon boiler geometry, providing short cool down times following emergency shutdowns. Also, an unstable bed size is related, in general, to less stable boiler operations, leading potentially to dangerous blackouts.
The present knowledge of the effect of bed inventory and distribution on the operations of the recovery system as a whole is enough to suggest that a constant bed inventory is essential to maintaining stable boiler operations. To achieve this goal, a clear continuous visual image of the bed would provide much of the information needed to permit improvement of control of the bed and, hence, improved chemical recovery.